A typical battery includes one or more electrochemical cells to store electrical energy. Each electrochemical cell includes an anode (negatively charged electrode during discharge of the cell), a cathode (positively charged electrode during discharge of the cell), an electrolyte between the anode and the cathode, and also typically includes a separator between the anode and cathode to, among other things, keep the anode and cathode from contacting each other.
An amount of electrical charge an electrochemical cell can store is related to the electrochemical system, which is a combination of reactive and nonreactive materials, an amount of electrode material and/or electrolyte material available for an electrochemical reaction. Generally, the greater the amount of available electrode and/or electrolyte material, the greater the charge capacity. In addition, larger electrode surface area decreases the internal resistance of the battery and can improve diffusion processes, which enables discharging and charging the battery at relatively large currents and improves other charge and discharge properties of the cell. Techniques to provide electrochemical cells with additional electrode surface and thereby improve cell performance include winding layers of the cell into a cylindrical shape to form a wound cell and stacking multiple layers of cells on top of one another to form a stacked cell.
Wound electrochemical cells are typically formed by layering anode, separator, and cathode layers adjacent each other, e.g., from continuous rolls of the respective layers, and then winding the layers to form a cylindrical structure. The cylindrical structure can be flattened to form a flat pack structure, which may better conform to design configurations of devices that use the batteries including the cells. Because the wound cells can be formed from continuous rolls of materials, manufacturing wound electrochemical cells is a relatively inexpensive way to form electrochemical cells having relatively high charge capacity and other desired properties. However, wound electrochemical cells and batteries including the cells may experience an inhomogeneous distribution of pressure and force caused by a volume change of portions of the cell during charge and discharge of the cell; this is especially true when a wound cell is compressed into a flat pack. This change in pressure may reduce the performance of the battery, the safety of the battery, and/or the lifetime of the battery.
Stacked cells are formed by placing multiple structures, each including an anode, separator, and cathode layer, in a vertical stack. Compared to would cells, stacked cells are relatively expensive to manufacture, because pre-cut or formed sheets of the anode, separator, and cathode layers must be separately formed and then stacked upon one another, which requires time-consuming, precise alignment of the layers. In addition, the equipment required to precisely place each layer is relatively expensive. However, cells and batteries formed using this technique exhibit relatively homogeneous force distribution caused by any volume change of the cell during charge and discharge of the cell. Thus, such cells may exhibit increased performance, lifetime, and safety compared to similar cells formed using wound cell technology.
Another technique used to form electrochemical cells includes using a z-fold or accordion fold of one or more layers of the electrochemical cell. Using a z-fold technique may be advantageous compared to winding layers of a cell, because folding techniques may allow for more homogeneous pressure and force distribution within the cells; however, the equipment and time required for folding cell layers is generally greater than for winding the cell layers. Folding techniques may be advantageous over stacking methods, because at least some of the layers of cells can be derived from continuous or semi-continuous sheets of materials, whereas all layers of a stacked cell are pre-cut; however, the pressure distribution within a cell including folded layers may not be as uniform as within stacked cells.
U.S. Publication No. 2012/0208066 A1, published Aug. 16, 2012, in the name of Schaefer et al., discloses a z-fold technique used in forming an electrode stack of an electrochemical cell. The disclosed method includes a continuous layer of z-folded separator material and cathode and anode electrode plates that are interposed between z-folded layers of the separator material. Although the electrochemical cells disclosed in Schaefer et al. have some advantages over purely stacked electrochemical cells, the cells of Schaefer et al. still require precise formation and alignment of both anode and cathode plates of the cells.
PCT Publication No. WO 2009/078632 A2, published Jun. 25, 2009, in the name of LG CHEM., LTD., discloses a battery that includes a plurality of overlapping electrochemical cells, wherein each cell includes a cathode, an anode, and a separator, and a continuous separator sheet is disposed between the overlapping electrochemical cells. While the disclosed cells have the advantage of being surrounded by a continuous sheet of separator material, the cells still require precise formation and alignment of the cathode, separator, and anode plates on top of the continuous sheet of separator material.
JP Publication No. 09017441 A, published Jan. 17, 1997, in the name of Kazuhiro, discloses a square battery having a z-folded anode layer and a z-folded cathode layer, wherein the cathode layer is directly coated with a continuous coating of separator material. The battery also includes a current collector that extends vertically and horizontally to prevent the polar sheets from shifting. The current collector is purported to have an advantage of not requiring tabs on electrodes. However, the current collector disclosed in Kazuhiro adds considerable weight and volume to the battery. In addition, the coated cathode and cells including such cathodes are thought to be relatively difficult to manufacture.
Although z-fold or accordion fold techniques for various layers within an electrochemical have been developed, the techniques still include additional steps, alignment of multiple plates, relatively difficult manufacturing steps, and/or add additional volume and weight to the cell. Accordingly, improved electrochemical cells and batteries and methods of forming the cells and batteries are desired.